Fluid sample filtration device

ABSTRACT

The present invention relates to a sample filtration device of the type employing differential pressure. An outer container filled with a sample to be filtered slidably receives a hollow plunger having filter media disposed near one end and sealing means disposed in an annular groove about the periphery of the plunger. The annular groove of the plunger is formed by two components: an annular shoulder on a collector portion and the axial face of an annular ring formed in a retainer portion. The two portions are frictionally engaged to retain the filter in place and form the annular groove for the sealing means. This construction permits straight-pull molding of the component parts which eliminates mold mismatch flaws.

This is a continuation of application Ser. No. 148,260, filed Jan. 25,1988, now U.S. Pat. No. 4,891,134.

BACKGROUND OF THE INVENTION

This invention relates to differential pressure filtration devices and,in particular, to a differential filtration device having an improveddesign which permits manufacturing with straight-pull molds and withoutmold mismatch flaws which arise from side action molds and can causeleakage.

Filtration devices employing differential pressure have been previouslydescribed in many patents. See, for example, Farr U.S. Pat. No.3,481,477; Grover U.S. Pat. No. 3,693,804; Farr U.S. Pat. No. 3,969,250;Ahlstrand et al. U.S. Pat. No. 3,970,565; and Jaffe U.S. Pat. No.4,035,150.

A typical prior art device (FIGS. 1 and 2) includes an outer container(a) which slidably receives a hollow plunger (b). A filter (c) isdisposed near the end of the hollow plunger (b) and is retained there bya retainer ring (d) ultrasonically welded to the inside of the hollowplunger (b). Typically, an O-ring (e) is disposed in an annular groove(f) circumscribing the hollow plunger (b). The annular groove (f) isbounded and defined by shoulder portions (g) and (h) of increaseddiameter which prevent the O-ring (e) from moving with respect to thehollow plunger (b).

A major drawback of filtration devices of this type is the inability tomanufacture them on straight-pull molds due to the annular groove (f)which makes it impossible to remove this part from a straight-pull mold.Consequently, previously known filtration devices have been manufacturedin side action or two-part mold cavities divided longitudinally in half.As a result of two-part molds, mismatches known as parting lines (s) areinevitable along the seam joining the two halves. Even though mismatchflaws may only be on the order of a few thousandths of an inch, this canbe enough to cause the O-ring (e) to seat improperly in the annulargroove (f) and cause a leak. While flaws of this magnitude qenerallywill not permit liquids to pass, they often will permit air to pass,causing a poor seal. A poor air seal compromises the pressurization oftrapped air essential to good sample filtrate delivery in differentialfiltration devices.

In addition, heat generated by the ultrasonic welding of the retainingring (d) to the end of the hollow plunger (b) can often damaqe thefilter (c) which is adjacent thereto. Moreover, the pressure with whichthe ring (d) compresses the filter (c) cannot be adequately controlled.Insufficient or non-uniform pressure permits bypass leaks, whileexcessive pressure can damage the filter.

SUMMARY OF THE INVENTION

The present invention overcomes these disadvantages of the prior artfiltration devices by providing a sample filtration device manufacturedon straight-pull molds to avoid mismatch flaws associated with the priorart devices. The invention also overcomes the disadvantages ofultrasonic welding by providing friction means for retaining the filtermedia within the core of the hollow plunger.

In one aspect, the present invention comprises an improved differentialpressure sample filtration apparatus. An outer container, closed at oneend for holding a fluid sample for filtering, slidably receives a hollowplunger. The hollow plunger comprises two pieces: a first piece orcollector portion having an annular shoulder thereon; and a second pieceor retaining portion having a first annular ring. The retaining portionis affixed to the collector portion such that the annular shoulder ofthe collector portion and the first annular ring of the retainingportion cooperate to form the sides of an annular groove which retains asealing means. The sealing means may comprise any structure capable ofachieving a fluid seal between the outer container and the hollowplunger, for example, an O-ring or flat washer. A filtering means isdisposed near one end of the plunger for filtering fluid from the outercontainer into an inner collecting means in the hollow plunger, as theplunger is inserted into the outer container.

Preferably, the retaining portion also includes a plurality of legsconnecting the first annular ring to a second annular ring which issmaller in diameter and axially spaced from the first annular ring. Thesecond annular ring is dimensioned to fit inside the hollow portion ofthe plunger to retain the filter media in place. Ideally, the retainingportion is affixed to the collector portion by means of friction fit or"snap fit" between the second annular ring and the interior of thehollow plunger.

In another aspect, the invention comprises a method for forming andassembling the components of the filtration device using straight-pullmolds, which method eliminates leaking due to parting line mismatchflaws. According to the method of the invention, a collector portion ofthe plunger and a retaining portion of the plunger are formedseparately, each from a distinctly configured straight-pull mold, andare assembled so that an annular shoulder on the collector portioncooperates with an annular face on the retaining portion to form theannular groove in which the sealing means is disposed. Preferably, theretaining portion and the collector portion remain engaged due tofriction between a second annular ring dimensioned to frictionallyengage the interior of the collector portion.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the invention can be had by reference to thefollowing detailed description of a preferred embodiment when read inconjunction with the accompanying drawings in which like referencenumerals refer to like parts throughout the several views and in which:

FIG. 1 is a longitudinal cross-section of a filtration device known inthe prior art;

FIG. 2 is an exploded perspective view of the plunger portion of thefiltration device of FIG. 1;

FIG. 3 is a longitudinal cross-sectional view of the filtration deviceof the present invention;

FIG. 4 is an exploded perspective view of the plunger portion of theinvention;

FIG. 5 is a cross-sectional view taken substantially along line 5--5 ofFIG. 3;

FIG. 6 is a bottom plan view of the retaining portion of the plunger;and

FIG. 7 is a diagrammatic representation of a straight-pull mold employedin the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 3, the filtration device 10 is depicted in longitudinalcross-section and comprises a cylindrical outer container 12 into whichis slidably received a cylindrical, hollow plunger. The hollow plungercomprises a first piece or collecting portion 14, the upper end of which(as viewed in FIGS. 3 and 4) is closed by a cap 16. A second piece ofthe plunger, a retaining portion 18 (somewhat resembling and hereinreferred to as a "crown"), is shown at the opposite end of the collectorportion 14.

As shown in FIGS. 3 and 5, the outer container 12 comprises acylindrical tube having inner and outer walls. The container 12 is openat a top end and closed at a bottom end. A section of the container 12near the bottom end preferably includes a radially reduced portion 19and the interior face of the bottom end includes a raised bump 20 for areason to be subsequently discussed. Circumferentially spaced in theinterior wall of the container 12, just above the reduced portion 19,are formed a plurality of raised nubs 21 which serve a purpose to bedescribed below.

The collector portion 14 (FIGS. 3, 4 and 5) similarly comprises acylindrical tube but is open at both ends to form a hollow passageway orcollecting chamber in the interior of the plunger. The exterior of thelower or inserted end (as viewed in FIGS. 3 and 4) of the collectorpiece 14 also has a reduced diameter portion 22 forming an annularshoulder 23. The interior lower end of the collector portion 14 housesfilter media 24 and may or may not have a correspondingly reducedportion.

The filter media 24 may be single or multiple layer and comprises anyknown filtering media including, but not limited to, paper, glass fiber,cellulose, and nitrocellulose. Depth filters are generally preferredover membrane filters due to their ability to remove greater quantitiesof particulate matter without becoming occluded. Especially preferredfor most applications is fiberglass combined with polypropylene,however, different media may be preferred, depending on the application.

The filter media 24 is held in place from above by an annular ledge 26formed in the interior walls of the collector portion 14 and,optionally, by cross bars 28 extending diametrically across the openingformed by the annular ledge 26. The annular ledge 26 and, if necessary,the cross bars 28 support the filter media 24 against the pressurizedsample which must be filtered as the plunger is inserted into the outercontainer 12.

From below, the filter media 24 is supported by the retaining portion orcrown 18. As best shown in FIGS. 4 and 6, the crown 18 comprises a firstannular ring 30 and a second annular ring 32 which are joined togetherby a plurality of longitudinal legs 34. The first annular ring 30 islarger in diameter and is dimensioned to fit snugly over the reduceddiameter portion 22 of the collector piece 14. Conversely, the secondannular ring 32 is smaller in diameter and is dimensioned to fit snuglyinside the interior wall of the reduced portion 22 of the collectorpiece 14. Accordingly, the legs 34 are somewhat L shaped. As best seenin FIGS. 3 and 4, the legs 34 axially space the second annular ring 32from the first annular ring 30 which gives the crown 18 itscharacteristic appearance. Furthermore, the second annular ring 32includes a portion that extends axially upward from the foot of the leg34 so that it can be inserted into the hollow interior of the reduceddiameter portion 22 of the collector portion 14 to retain the filtermedia 24 in position. Preferably, circumferentially spaced raised nubs33 are disposed on either the outside of the second annular ring 32 (seeFIG. 4) or the inside of the reduced portion 22 below the annular ledge26 (not shown). The other component (ring 32 or inside of portion 22)has depressions (not shown) corresponding to the nubs 33 to securelylock one to the other.

As best seen in FIG. 3, the legs 34 have a length slightly less than theaxial length of the reduced diameter portion 22 which leaves a gapbetween the top axial face 36 of the first annular ring 30 and theannular shoulder 23 formed by the reduced portion 22 of the collectorpiece 14. The gap defines an annular groove about the periphery of theplunger into which an O-ring 40 is securely seated. In this manner, theO-ring 40 slides with the plunger as it is inserted into the outercontainer 12 so that a fixed quantity of air is trapped in the outercontainer 12 and forces the sample through the filter media 24.

While the preferred embodiment of the crown 18 has been described, theinvention also contemplates other straight-pull molded retainingportions secured to a collector portion 14 by suitable means and havingan annular face axially spaced from the annular shoulder 23 to form anannular groove. A second ring, distinct or continuous with the retainingportion, can optionally form the filter retaining means.

It will be apparent to those skilled in the art that the gap between theannular shoulder 23 and the top axial face 36 is equivalent in functionto the annular groove (f) of the prior art defined by the raisedportions (g) and (h) as shown in FIGS. 1 and 2. However, by forming thegap from two components (i.e. the collector portion 14 and the crownportion 18 of the plunger), it is possible to achieve the filtrationdevice 10 which can be molded from straight-pull molds to eliminatemismatch flaws or parting lines inherent in two-piece or side actionmolds. By eliminating these mismatch flaws, it is possible to decreasethe chances that a particular plunger will have an air leak past theO-ring 40.

Each of the major components of the device 10, namely the outercontainer 12, the collector piece 14 and the crown 18, is made of arelatively rigid substance such as plastic, polypropylene orpolyethylene. These components may all be made of the same material. Anespecially preferred substance is polypropylene because it is easilymolded by injection molding techniques and relatively inert to theassay. Ideally, the material is flexible enough and the dimensions smallenough to permit the nubs 21 to be formed in spite of the shear actionof the mold.

A typical mold (FIG. 7) for forming the collector portion 14 of theplunger comprises a cavity 50 cut into a block 52. The cavity 50 isdimensioned to correspond to the outer wall of the collector portion andis open at the top. The bottom of the cavity 50 can be formed by theblock 52, but more conveniently is formed by a pin 53 inserted into thecavity 50 to a predetermined position. The block 52 also has a port orgate 54 opening between the cavity 50 and a source 56 of molten plasticto permit injection molding of the component. A core 58 is inserted intothe center of the cavity 50 and is dimensioned to correspond with theinterior dimensions of the collector portion 14. The core 58 is centeredwithin the cavity 50 by means of spacers 60 which also can be used toremove the formed collector piece 14 from the central core 58 by sharpdownward pressure. The pin 53 is centered by a spacer plate or collar62, which may be integral with the block 52 or separate.

The crown 18 can be formed similarly from a straight-pull mold having acavity and central core slightly different than those of FIG. 7 butwhich can easily be determined by one skilled in the art. The retainingportion or crown 18 is then slid over the reduced diameter portion 22 ofthe collector portion 14 and locked into place by frictional engagementof the nubs 33. The retainer portion forms part of the annular groovefor the O-ring 40 and, simultaneously, holds the filter media 24 inplace.

In use, a liquid sample to be filtered is placed in the bottom of thereduced diameter portion 20 of the outer container 12. The plunger isinserted filter end first into the open end of the outer container 12and the O-ring 40 sealingly engages the inner wall to form an air-tightseal between the outer container 12 and the plunger 14. As the plungeris depressed further into the outer container 12, air is forced throughthe filter media 24 and escapes through the loosely fitting cap 16.

Once the plunger reaches the surface of the sample fluid to be filtered,a fixed quantity of air is trapped between the O-ring 40 and the fluidlevel and, upon further depression of the plunger this trapped air iscompressed. The pressurized air in turn forces the fluid sample throughthe filter media 24 and into the collecting chamber in the interior ofthe hollow plunger. As the reduced diameter portion 22 of the plunger ispushed into the reduced diameter portion 20 of the outer container 12,the trapped quantity of air is forced into a smaller volume whichmultiplies its compressive effect on the fluid to deliver as much sampleas possible through the filtering media 24. This arrangement increasesthe efficiency of filtration which is important for small volumesamples.

The bump 20 at the bottom end of the outer container 12, by occupyingspace inside the second annular ring 32, also serves to decrease theavailable space for trapped air, thereby to deliver as much samplefiltrate as possible.

Filtration is complete when the plunger hits the bottom end of the outercontainer 12. Simultaneously, the O-ring 40 snaps past the nubs 21formed in the inner wall of container 12 to lock the two componentstogether. This feature permits thick and thin samples alike to befiltered without the need for holding the components together manually.This is advantageous since some samples take longer than others tocompletely filter.

When filtration is complete, the filtrate can be poured from the openend of the hollow plunger upon removal of the cap 16. Alternatively,additional reagents can be added to the filtered sample in the innercollector portion to further process the sample prior to pouring.Filtered sample can then be poured into any desired assay format withoutincluding undesired particulate matter present in the original sample.

The foregoing description of the preferred embodiment has been given forpurposes of illustration only and no unnecessary limitations should beunderstood therefrom. Rather, the invention is intended to be limitedonly by the following claims.

What is claimed is:
 1. A differential pressure sample filtration devicecomprising:an outer container for receiving a fluid for filtering; and ahollow plunger dimensioned to be slidably received in said outercontainer and comprising:a first piece having at one end a reduceddiameter portion forming an annular shoulder, means for collecting fluidfiltrate and filter means disposed between said one end of the plungerand said means for collecting fluid filtrate; and a second piece havingan annular axial face and being dimensioned to be secured to said firstpiece such that the annular axial face and the annular shouldercooperate to form an annular groove about said plunger, said annulargroove being adapted for receiving a sealing means for forming a fluidseal between the outer container and the hollow plunger, wherein saidsecond piece comprises a first annular ring dimensioned to slidably fitover said one end of said first piece, a second annular ring dimensionedto slidably fit into the hollow portion of said one end of the firstpiece to abut said filter means, and a plurality of legs connecting saidfirst annular ring to said second annular ring.
 2. The device accordingto claim 1 wherein said first and second pieces of the plunger arelockingly engaged by friction.
 3. The device according to claim 1wherein the annular axial face includes a frusto-conical aspect.
 4. In asample filtration device of the differential pressure type having ahollow plunger slidable into an outer container of fluid to be filtered,a filter disposed in one end of said plunger, and sealing means disposedin an annular groove about the plunger to sealably engage said outercontainer wall, the improvement comprising:a plunger having a collectorportion and a retaining portion, wherein said collector portion includesan annular shoulder and said retaining portion includes a first annularring dimensioned to slidably fit around said plunger, said annularshoulder and said annular ring defining said annular groove; and whereinsaid retaining portion further comprises a second annular ringdimensioned to slidably fit into said one end of the hollow plunger toabut said filter, and leg means rigidly connecting said first and secondannular rings.
 5. The device according to claim 4 wherein said firstannular ring includes a frusto-conical face.